The present invention relates to web transport systems, and more particularly to apparatus for controlling the tension of a web during transport.
One well known type of decorating apparatus, heat transfer labelling apparatus, transfers decorative labels from a carrier web to articles using heat and pressure. As shown in the prior art plan view of FIG. 1, a typical heat transfer decorator 100 includes apparatus 105 for transporting a label carrier web 150 to a decoration station 160, where an individual label is transferred from the web to an article B by means of a transfer roll 146. In the illustrative web transport 105, the label-carrier web travels from a supply reel 110, around a dancer roll 120, various idler rolls, a metering roll 10, a label preheater 143, a platen and transfer roll 146, a takeup dancer roll 140, to a takeup reel 115. One of the operational prerequisites of the labelling apparatus 100 is that the carrier web 150 be maintained at a controlled, even tension in order to provide smooth web transport, as well as reliable web tension at the decoration site 160.
FIG. 2 shows in a perspective view an advantageous form of tension control apparatus as known in the prior art. The carrier web 150 unwinds from supply reel 110 (shown in phantom), to a dancer roll 120, which moves in an arcuate path in response to tension differentials. The unwind dancer roll 120 is mounted on a dancer lever 122, which in turn is pivotally mounted to a connecting rod 123; the pivot point is assymetrically located, so that the pivoting of lever 122 will induce the axial translation of connecting rod 123. The translation of connecting rod 123 in turn varies the compression of a spring (not shown), thus applying greater or less force to pivoted clutch arm 125. As shown in FIG. 3, clutch arm 125 is pivotally mounted to mounting block 126. The greater the force exerted through clutch arm 125, the greater the engagement pressure between the lower and upper clutch plates 127 and 128, and the greater the frictional restraining torque imposed by the clutch 130. Clutch arm 125 may be rotatably positioned by adjustment bolt 129. This adjustment is used to properly position the range of arcuate travel of the dancer rolls.
The motion of the various mechanisms linked to dancer roll 120 results in variation of the engagement of the clutch 130, which determines the frictional resistance exerted by the clutch to the rotation of supply reel 110, and hence the web tension required to slip the clutch. For example, movement of dancer roll 120 away from decoration site 160 causes connecting rod 123 to push outwardly on clutch arm 125, increasing the pressure between clutch plates and increasing the resisting torque. To maintain a proper tension balance, the resisting torque of the clutch should balance the torque exerted by the web at supply reel 110, which varies with the radius of the supply roll. During normal operation, the dancer roll 120 wil tend to move away from decoration site 160 when the supply roll radius is high, and toward the decoration site 160 when the supply radius is low.
The mechanisms illustrated in FIGS. 2 and 3 are duplicated at takeup reel 115, with the modification that the takeup reel is mounted to a positive drive (not shown) for the web transport. The takeup reel is connected to the positive drive by a slip clutch (not shown) analogous to the braking clutch 130; engagement pressure between the clutch plates at the takeup reel is controlled in similar fashion as at the supply reel 110 thus maintaining proper web tension at the supply reel.
It is desirable to minimize any tension difference across the metering roll 10. This prior art system utilizes a single air cylinder (not shown) to regulate the tension at each dancer lever. For example, in the supply linkage the air cylinder is linked by a lever (not shown) to the pivot connection of dancer lever 122 and connecting rod 123, providing a counterbalance to the torque exerted by the dancer lever 122 (FIG. 2). In order to minimize tension difference across the metering roll, the lengths of the cylinder lever in the supply linkage and corresponding lever in the rewind linkage are relatively adjusted to compensate for the various frictional restraints imposed in the web by idler rolls, contact with heaters, and the like.
A disadvantage of this prior art apparatus is that the mechanical adjustments provided do not automatically compensate for tension variations which occur for various reasons. This has led to a degradation of decoration quality, and in addition has caused mechanical problems in web transport. In particular, a tension differential at the web metering roll 10 can cause a fracture of the web at the feed holes 157.
Accordingly, it is a principal object of the invention to provide an improved web transport system of the general type illustrated in the prior art. A related object is to provide improved tension control apparatus for use in a transport system of this type.
Another object of the invention is to achieve tension control apparatus which effectively compensates for changes in transport parameters during the operation of the transport system. A specific object is that such a tension control system effectively compensate for tension differentials during the unwind and takeup of the web.
A further object of the invention is to provide a web transport system which is easily adjustable by the user, but which does not require constant user surveillance.
Yet another object of the invention is to achieve web tension control apparatus which minimizes mechanical stresses on the web during normal operation.